x PREFACE 



calling my attention to publications that otherwise 

 might have escaped my notice. I may, however, be 

 allowed to say that certain typically German compila- 

 tions such as those of ^Novak, Seitz, and Hofstatter had 

 not reached this country when my material was collected ; 

 and, further, that concerning a subject whose foundations 

 rest on the basal processes of life individual metabolism 

 and reproduction the literature must necessarily be too 

 extensive for detailed consideration except in its most 

 immediate bearings. 



This is not the kind of book that can usefully be 

 taken through numerous editions, so with the issue of 

 this edition it will finish its career. If the work has 

 succeeded in defining and focussing attention on the 

 subject, and in provoking emulative criticism, the 

 purpose of it will have been fully served. 



Once more I have to thank my friend Dr. Hubert 

 Armstrong for his careful correction and criticism of the 

 proof-sheets, and for making the index. 



W. B. B. 



38, Rodney Street, 

 Liverpool. 



June, 1920. 



